Apparatus for detecting a state of a particulate filter

ABSTRACT

A particulate filter is provided in an exhaust system of the engine. Parameter values correlated with a flow amount of exhaust gas of the engine are determined, and differential pressure between upstream and downstream sides of the particulate filter is determined. The ratio of a variation rate of the parameter values correlated with the exhaust flow amount to a variation rate of the differential pressure is compared with a predetermined threshold value during transitional operation of the engine to determine the state of the particulate filter.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for detecting a state of aparticulate filter for trapping particle matters in exhaust gas of aninternal-combustion engine.

In general, an engine, in particular, a diesel engine exhausts particlematters (herein after referred to as “PM”) at its operation time. Inorder to prevent the PM from exhausting to the atmosphere, a particulatefilter is provided in an exhaust system of the diesel engine. Theparticulate filter traps the PM when the exhaust gas passes throughsmall holes in a wall of the filter toward adjacent passage.

Sometimes, the particulate filter may not be able to trap the PM due tosome damage in the filter. In order to detect such a failure state, itis preferable that the state of the particulate filter be alwaysestimated from sensor information.

The Japanese Patent Application Publication No. S57-159519 discloses atechnique for determining clogging in the particulate filter bydetecting a difference between pressures in upstream and downstreamsides of the particulate filter and an exhaust flow amount.Determination of clogging is made based on the ratio of the two values.

Japanese Patent Application Publication No. H8-284644 discloses atechnique for calculating an amount of PM accumulated in a particulatefilter based on the difference between pressures in upstream anddownstream sides of the particulate filter and a flow volume of exhaustgas. Based on the accumulated PM amount, time for starting aregeneration process on the particulate filter is determined. Also,failure of the particulate filter is determined based on change in timeof the accumulated PM amount.

Characteristics of the state of the particulate filter appear clearlyduring transitional operation such as acceleration and deceleration. Itis an objective of the present invention to provide a scheme fordetermining a state of a particulate filter during transitionaloperation of an engine.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for detecting a state of theparticulate filter of an internal-combustion engine. The particulatefilter is provided in an exhaust system of the engine. The apparatusincludes means for detecting values of a parameter correlated with theflow amount of exhaust gas of the engine. The apparatus also includes adifferential pressure detecting means for detecting the differentialpressure between upstream and downstream sides of the particulatefilter. The apparatus further include a means for determining failure ofthe particulate filter by comparing with a predetermined threshold valueduring transitional operation of the engine the ratio of a variationrate of the parameter correlated with the exhaust flow amount and avariation rate of the differential pressure.

According to the invention, the state of the particulate filter can beaccurately determined during transitional operation of the engine whencharacteristics of the state of the particulate filter clearly appear.

According to one aspect of the present invention, the parameter having acorrelation with the exhaust flow amount is an internal pressure of anair intake pipe of the engine.

According to another aspect of the present invention, the ratio of thevariation rate of the parameter correlated with the exhaust flow amountto the variation rate of the differential pressure is compared with afirst threshold value to determine a failure of the particulate filter.Further, the ratio of the variation rate of the parameter correlatedwith the exhaust flow amount to the variation rate of the differentialpressure is compared with a second threshold value to determineover-accumulation of particulates in the particulate filter.

According to a further aspect of the present invention, the firstthreshold value is established in accordance with an accumulation amountof the particulates in the particulate filter and the second thresholdvalue is established in accordance with an amount of residue ashes inthe particulate filter. According to this invention, the state of theparticulate filter can be accurately determined taking intoconsideration of influences of the amount of the accumulated PM and theamount of the residue ashes.

According to a yet further aspect of the present invention, the statedetermining means integrates differences between a current value and aprevious value of the ratio of the variation rate of the parametercorrelated with the exhaust flow amount to the variation rate of thedifferential pressure. The particulate filter is determined to be infailure when the integrated value of the differences is equal to orlarger than a predetermined threshold value. According to the invention,failure of the particulate filter can be accurately detected based onthe characteristic of the variation rate of the differential pressurewhen the particulate filter has a trouble.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a structure of a diesel engine mountedon a vehicle and its control unit in accordance with one embodiment ofthe present invention.

FIG. 2 is a graph showing alteration of intake manifold pressure and DPF(diesel particulate filter) differential pressure when an engine isaccelerated.

FIG. 3 is a graph showing a behavior and a variation rate β of DPFdifference pressure in accordance with various DPF states during engineacceleration under the same condition.

FIG. 4 is a graph showing a first threshold value γa and a secondthreshold value γb in a first embodiment of the present invention.

FIG. 5 is a flowchart of a process for detecting a state of the DPF inaccordance with a first embodiment of the present invention.

FIG. 6 is a graph showing a first threshold value γa(pm) and a secondthreshold value γb(ash) in a second embodiment of the present invention.

FIG. 7 is a flowchart of a process for detecting a state of the DPF inaccordance with a second embodiment of the present invention.

FIG. 8 shows movements of the values of γ which are calculated for eachcycle of DPF state detecting processes at DPF normal time and at DPFfailure time respectively.

FIG. 9 is a flowchart of a process for detecting a state of the DPF inaccordance with a third embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of an apparatus for detecting a state of aparticulate filter in accordance with the present invention will bedescribed below with reference to the accompanying drawings. FIG. 1 is ablock diagram showing a structure of a diesel engine mounted on avehicle and its control unit in accordance with one embodiment of thepresent invention.

A diesel engine 11 is a direct injection type of engine in which fuel isinjected into a combustion chamber of each cylinder and compressed fornatural ignition. The diesel engine 11 controls its output by adjustingfuel injection amount and injection timing by injectors (not illustratedin the drawings) attached to each cylinder. Each injector injects fuelat an optimum timing based on a control command an electric control unit(ECU).

The ECU 13 includes an input interface 13 a for receiving data fromvarious parts of a vehicle, and a CPU 13 b for performing computationsfor controlling various parts of the vehicle. ECU13 also includes amemory 13 c including a Read-Only Memory (ROM) and a Random AccessMemory (RAM). ECU 13 further includes an output interface 13 d forsending control signals to various parts of the vehicle. The ROM storescomputer programs and data required for controlling various parts of thevehicle. Programs to be used for detecting a state of a particulatefilter according to the present invention are stored in the ROM. The ROMmay be a re-writable type of ROM such as an EPROM. The RAM provides aworking space for CPU 13 b. The data transmitted from various parts ofthe vehicle and the control signals sent out to various parts of thevehicle are temporarily stored in the RAM.

Various signals including sensor outputs transmitted to the ECU 13 arereceived by the input interface 13 a in which the signals are convertedfrom analog to digital. The CPU 13 b processes the converted digitalsignals according to the programs stored in the memory 13 c to createcontrol signals to be sent to various parts of the vehicle. The outputinterface 13 d sends the control signals to various elements of theengine for controlling the operation of the engine.

A diesel particulate filter (DPF) is provided in an exhaust pipe 15 ofthe diesel engine 11. The DPF 17 consists of a heat-resistant porousfilter wall formed by ceramics, non-woven metallic fiber cloth or thelike. The DPF 17 has a plurality of passages forming exhaust flowpassages in a flowing direction of the exhaust gas. The diameter of apore is about 10 microns, so that the particle matters (PM) contained inthe exhaust gas can be trapped when they pass through the porous wall.

One end of the exhaust passages of the DPF 17 is blocked either atupstream side or at downstream side in the flowing direction of theexhaust gas. The passages whose upstream end are blocked and thepassages whose downstream end are blocked are arranged alternatively andadjacently to each other. Accordingly, the exhaust gas discharged froman exhaust port of each cylinder flows into the passage whose upstreamend is open with the downstream end closed, passes through the porouswall separating the adjacent passages each other into the passage whosedownstream end is open, thus exiting the DPF from the downstream end.The DPF 17 needs a regeneration process periodically because the PMtrapped in the DPF may cause deterioration of filter performance whenthe PM over-accumulates in the filter.

The PM trapped in the DPF 17 are removed from the DPF by raising thetemperature of the filter high enough to burn the PM. A well-knowntechnique for raising the exhaust gas temperature such as postinjection, closing of intake shutter valve, EGR introduction or the likemay be used to raise the temperature of the filter.

A differential pressure sensor 19 is connected to both of upstream anddownstream sides of the DPF 17 via a pressure introducing pipe. Thedifferential pressure sensor 19 sends a signal indicating a differenceof pressure between the upstream and downstream sides of the DPF 17 tothe ECU 13. The differential pressure between the upstream anddownstream sides of the DPF 17 has a characteristic that it increases inaccordance with increase in the amount of the PM accumulated in thefilter. The ECU 13 can estimate a PM accumulation amount in the DPF 17by utilizing the output of the differential pressure sensor 19.Furthermore, the ECU 13 can estimate an amount of ashes remaining in theDPF 17 based on the output of the differential pressure sensor 19immediately after the regeneration process.

A pressure sensor 23 is provided in an air intake pipe 21 of the dieselengine. In this embodiment, the pressure sensor is disposed in an intakemanifold (not illustrated in the accompanying drawings) in the airintake pipe and sends to the ECU 13 an output signal showing intake airpressure in the intake manifold (such pressure will be hereinafterreferred to as an “intake manifold pressure”). In addition, othersensors and devices such as a turbo-charger and a common-rail and thelike for operating the diesel engine 11 are provided in the enginethough those devices are not illustrated in the accompanying drawings.

Now, a technique for detecting a state of the DPF according to a firstembodiment of the present invention will be described below.

FIG. 2 is a graph showing alteration of the intake manifold pressure 31and the DPF differential pressure 33 when the engine 11 is accelerated.The intake manifold pressure 31 is measured by the pressure sensor 23provided in the air intake pipe 21. The DPF differential pressure 33 ismeasured by the differential pressure sensor 19 connected to upstreamand downstream sides of the DPF 17.

When the engine 11 is accelerated, an amount of the air flowing into theair intake pipe increases as larger amount of air is sucked into thecombustion chamber of the engine 11. Accordingly the intake manifoldpressure 31 increases. Time required for increasing the intake manifoldpressure 31 from A1 (time t1) to A2 (time t2) is expressed by t2−t1.Variation rate a of the intake manifold pressure 31 during that timeperiod can be calculated according to Equation (1).α=(A2−A1)/(t2−t1)  (1)where α can be expressed as a gradient of a straight line 35 connectinga coordinate (t1, A1) with a coordinate (t2, A2) in FIG. 2.

In proportion to an increase of the intake air amount, the flow amountof the exhaust gas within the exhaust pipe 15 increases and the DPFdifferential pressure 33 increases as well. Besides, alteration of theDPF differential pressure 33 has a time delay At relative to alterationof the intake manifold pressure 31 in accordance with a positionalrelation between the pressure sensor 23 for measuring the intakemanifold pressure and the differential pressure sensor 19 for measuringthe DPF differential pressure. A variation rate β of the DPFdifferential pressure 33 corresponding to a variation rate a of theintake manifold pressure 31 can be calculated according to Equation (2).β=(B2−B1)/(t2−t1)  (2)where B1 represents a DPF differential pressure at time t1+Δt and B2represents a DPF differential pressure at time t2+Δt. β is expressed asa gradient of a straight line 37 connecting a coordinate (t1+Δt, B1)with a coordinate (t2+Δt, B2) in FIG. 2.

The variation rate a of the intake manifold pressure and the variationrate β of the DPF differential pressure take different values inaccordance with the degree of acceleration of the engine. However,alteration of the behavior responsive to acceleration conditions is thesame for the two and there is a certain correlation between thevariation rate α of the intake manifold pressure and the variation rateβ of the DPF differential pressure.

In this embodiment, the state of the DPF is detected by using thevariation rate α of the intake manifold pressure and the variation rateβ of the DPF differential pressure during engine acceleration.

FIG. 3 is a graph showing difference in alteration of the DPFdifferential pressure in accordance with different states of the DPF 17during acceleration with the same condition. Since the engineacceleration condition is unchanged, the variation rate α of the intakemanifold pressure is constant.

A graph 33 a shows a DPF differential pressure in a state where theaccumulated amount of the PM is large to cause clogging in the DPF (thisstate will be hereinafter referred to as “over-accumulated state”).Under the over-accumulated state, the DPF differential pressure becomeslarge because the exhaust gas cannot flow smoothly through the DPF 17 incomparison with the normal time. Accordingly, the variation rate βbecomes large.

A graph 33 c shows a DPF differential pressure in a state where certainbreakage exists in the DPF (such state will be hereinafter referred toas a “defect state”). Under the defect state, the DPF differentialpressure becomes small because the exhaust gas flows more smoothlythrough the DPF 17 than in the normal state. Correspondingly, thevariation rate β becomes smaller.

Thus, the variation rate β of the DPF differential pressure has acharacteristic to have different values depending on the state of theDPF.

Now, a ratio γ=α/β of the variation rate a of the intake manifoldpressure to the variation rate β of the DPF differential pressure willbe considered below. When the DPF is in the defect state, the value of γis large because β, which is the denominator of γ, is small. On theother hand, when the DPF is in the over-accumulated state, the value ofγ is small because β, the denominator of γ, is large.

As can be seen in FIG. 4, γ lies in respectively different regions inaccordance with defect, normal and over-accumulated states of the DPF17. Therefore, by establishing appropriate threshold values, the stateof the DPF 17 can be classified into the defect state, the normal stateand the over-accumulated state. For example, referring to FIG. 4, thedefect state can be distinguished from the other states by a firstthreshold value γa. This state indicates that the DPF 17 is in failure.Similarly, the over-accumulated state can be distinguished from theother states by a second threshold value γb. This state indicates thatthe DPF 17 has accumulated more PM than an allowable amount.

In this embodiment, the state of the DPF 17 is determined based on theratio γ of the variation a of the intake manifold pressure to thevariation β of the DPF differential pressure during engine acceleration.

FIG. 5 is a flowchart of a process for detecting the state of the DPF 17in accordance with the present embodiment.

In Step S101, the intake manifold pressure and the DPF differentialpressure are measured. The intake manifold pressure 31 is measured bythe pressure sensor 23 provided in the air intake pipe 21. The DPFdifferential pressure 33 is measured by the differential pressure sensor19 connected to the upstream and downstream sides of the DPF 17.

In Step S103, the variation rate a of the intake manifold pressure iscalculated according to Equation (1) and the variation rate β of the DPFdifferential pressure is calculated according to Equation (2). In thisstep, a time lag Δt (to be used in Equation (2)) between the intakemanifold pressure and the DPF differential pressure is determined basedon the positional relation of the pressure sensor and the differentialpressure sensor and the intake air flow amount or the exhaust flowamount.

In Step S105, it is determined whether or not the variation rate a ofthe intake manifold pressure is equal to or larger than a predeterminedvalue. When the variation rate α is equal to or larger than thepredetermined value, it is determined that the engine is in atransitional operation condition and the process goes to Step S107. Whenthe variation rate a is smaller than the predetermined value, theprocess is exited.

In Step S107, the ratio γ=α/β of the variation rate a of the intakemanifold pressure to the variation rate β of the DPF differentialpressure is calculated.

In Step S109, it is determined whether or not γ is larger than apredetermined first threshold value γa. When γ is larger than thethreshold value γa, the DPF is determined to be in failure (S11). When γis equal to or smaller than the threshold value γa, the process goes toStep S113.

In Step S113, it is determined whether or not γ is larger than apredetermined second threshold value γb. When γ is larger than thethreshold value γb, the DPF is determined to be normal (S115). When γ isequal to or smaller than the threshold value γb, the process goes toStep S117, in which it is determined that the DPF is in anover-accumulated state and a regeneration process is performed.

In the present embodiment, as described above, detection of the DPFstate is carried out when the engine is accelerated. Since suchtransitional time as the engine acceleration time takes place frequentlyduring vehicle operation, detection of the DPF state in accordance withpresent embodiment can be performed frequently during vehicle operation.

Now, a second embodiment according to the present invention will bedescribed below.

The basic concept of the DPF state detecting technique according to thesecond embodiment is the same as the first embodiment, but the secondembodiment is different from the first embodiment in a method forestablishing threshold values to be used for determining the DPF state.

FIG. 6 shows a first threshold value γa(pm) and a second threshold valueγb(ash) used in the second embodiment. As described above with referenceto FIG. 4, the state of the DPF 17 can be determined based on a variablevalue of the ratio γ of the variation a of the intake manifold pressureto the variation β of the DPF differential pressure. However, as shownby a region 39 in FIG. 6, when some failure like a breakage takes placein the over-accumulated state, there can be a consequence that theexhaust gas does not flow through the DPF in the same manner as in thenormal state and the value of y does not become large in comparison withthe normal state. In such situation, the failure state of the DPF 17 maynot be distinguished as γ does not exceed the first threshold value γa,which is a constant.

Thus, in the second embodiment, the first threshold value to be used fordetermining failure of the DPF is set as a function γa(pm) that isvariable in accordance with the accumulation amount pm of the PM. Thefirst threshold value γa(pm) is set to become smaller in proportion toincrease of the PM accumulation amount pm as shown in FIG. 6.

After regeneration process is performed upon the DPF, some unburnedmatters (ash) of oil elements or the like included in the exhaust gasmay remain within the DPF. Such ashes accumulate within the DPF everytime the regeneration process is performed. The variation rate β to beused for determining the DPF state in the present embodiment isinfluenced by the amount of the residue ashes in addition to the PM. Inother words, when the amount of the residue ashes increases, thevariation rate β of the DPF differential pressure corresponding to thesame accumulation amount of the PM takes a larger value, which meansthat γ=α/β becomes smaller. If the second threshold value γb is aconstant, that the DPF may erroneously determined to be in theover-accumulated state due to the influence of the amount of the residueashes though the PM has not yet accumulated to such an extent as to needa regeneration process.

Therefore, in the present embodiment, the second threshold value to beused for determining the over-accumulated state of the DPF is set as afunction γb(ash) that is variable in accordance with the remaining ashamount ash. The second threshold value γb(ash) becomes smaller inproportion to increase of the remaining ash amount ash as shown in FIG.6.

FIG. 7 is a flowchart of a process for detecting a state of the DPF inaccordance with the second embodiment.

In Step S201, the intake manifold pressure and the DPF differentialpressure are measured. The intake manifold pressure 31 is measured bythe pressure sensor 23 provided within the air intake pipe 21 and theDPF differential pressure 33 is measured by the differential pressuresensor 19 connected to the upstream and downstream sides of the DPF 17.

The variation rate a of the intake manifold pressure and the variationrate β of the DPF differential pressure are calculated according toEquation (1) and Equation (2) respectively (S203). In this step, a timelag Δt (to be used in Equation (2)) between the intake manifold pressureand the DPF differential pressure is determined based on the positionalrelation of the pressure sensor and the differential pressure sensor aswell as the intake air flow amount or the exhaust flow amount.

In Step S205, the PM accumulation amount pm of the DPF and the remainingash amount ash are estimated. The PM accumulation amount pm iscalculated based on the intake manifold pressure, the DPF differentialpressure or the like according to known technique as described in, forexample, the above-referenced Japanese Patent Publication No. H8-284644.The remaining ash amount ash is calculated based on, for example, theDPF differential pressure immediately after a regeneration process hasbeen carried out. Alternatively, the residue ash amount ash may becalculated based on a difference Δγ between a ratio γ of the variationrate a of the intake manifold pressure with the variation rate β of theDPF differential pressure immediately after the regeneration process andan equivalent ratio γ regarding a thoroughly new or almost new DPF. Theresidue ash amount is calculated every time a regeneration process iscarried out and stored in the memory 13 c.

In Step S207, the first threshold value γa(pm) is set in accordance withthe estimated PM accumulation amount pm and the second threshold valueγb(ash) is set in accordance with the estimated residue ash amount ash.

In Step S209, it is determined whether or not the variation rate a ofthe intake manifold pressure is equal to or larger than a predeterminedvalue. When the variation rate α is equal to or larger than thepredetermined value, it is determined that the engine is in atransitional operation condition and the process moves to Step S211.When the variation rate a is smaller than the predetermined value, theprocess is exited.

In Step S211, a ratio γ=α/β of the variation rate a of the intakemanifold pressure to the variation rate β of the DPF differentialpressure is calculated.

In Step S213, it is determined whether or not γ is larger than the firstthreshold value γ a(pm). When γ is larger than the threshold valueγa(pm), the DPF is determined to be in failure (S215). When γ is equalto or smaller than the threshold value γa(pm), the process moves to StepS217.

In Step S217, it is determined whether or not γ is larger than thesecond threshold value γb(ash). When y is larger than the secondthreshold value γb(ash), the DPF is determined to be normal (S219). Wheny is equal to or smaller than the second threshold value γb(ash), theDPF is determined to be in the over-accumulated state (S221) and aregeneration process is performed.

In the present embodiment, as described above, since the thresholdvalues are changed in accordance with the PM accumulation amount and theremaining ash amount within the DPF, the DPF state can be detectedaccurately even in such state in which the PM and/or ashes areover-accumulated in the DPF.

Now, a third embodiment of the present invention will be describedbelow.

FIG. 8 shows behavior of the values of γ which are calculated for eachcycle of the DPF state detecting process during the DPF normal periodand the DPF failure period respectively. In the DPF normal period, thereis no significant change in the values of γ over many cycles as shown in(a) of FIG. 8. On the other hand, in the DPF failure period, as shown in(b) of FIG. 8, the variation rate β of the DPF differential pressurebecomes unstable because the flow of the exhaust gas in the DPF isunstable by the influence of defects. Accordingly, there is acharacteristic that unevenness of the values of γ which are calculatedfor different cycles of the process is large in the PDF failure period.

In the present embodiment, difference between the previous value and thecurrent value of γ is integrated for a predetermined number of times.When the integrated value exceeds a predetermined threshold value, theDPF state detection is performed. The integrated value represents adegree of unevenness of γ. In other words, larger integrated valueindicates larger unevenness of γ.

FIG. 9 is a flowchart of a process for detecting a state of the DPF inaccordance with the third embodiment.

In Step S301, the intake manifold pressure and the DPF differentialpressure are measured. The intake manifold pressure 31 is measured bythe pressure sensor 23 provided in the air intake pipe 21 and the DPFdifferential pressure 33 is measured by the differential pressure sensor19 connected to the upstream and downstream sides of the DPF17.

The variation rate a of the intake manifold pressure and the variationrate β of the DPF differential pressure are calculated according toEquation (1) and Equation (2), respectively (S303). In this step, a timelag Δt (to be used in Equation (2)) between the intake manifold pressureand the DPF differential pressure is determined based on the positionalrelation between the pressure sensor and the differential pressuresensor and the intake air flow amount or the exhaust flow amount.

In Step S305, it is determined whether or not the variation rate a ofthe intake manifold pressure is equal to or larger than a predeterminedvalue. When the variation rate a is equal to or larger than thepredetermined value, the engine is determined to be in a transitionaloperation condition and the process goes to Step S307. When thevariation rate a is smaller than the predetermined value, the process isexisted.

In Step S307, a ratio γ(t)=α/β between the variation rate α of theintake manifold pressure and the variation rate β of the DPFdifferential pressure is calculated.

In Step S309, a difference Δγ between the current value γ(t) and theprevious valueγ(t−1) regarding γ is calculated as in Δγ=γ(t)−γ(t−1).

In Step S311, it is determined whether or not a sum of squares of Δγ fora predetermined number of times, which is expressed as ΣΔγ², is largerthan a predetermined threshold value. When the sum of squares ΣΔγ² islarger than the threshold value, the DPF is determined to be in failure(S313). When the sum of squares ΣΔγ² is equal to or smaller than thethreshold value, the DPF is determined to be normal (S315).

The present invention has been described above with reference to somespecific embodiments. However, the present invention is not limited tosuch specific embodiments.

Although the intake manifold pressure is used for determining the stateof DPF in the above-described embodiments, any parameter may be used aslong as it has a correlation with the flow amount of the exhaust gas.For example, the present invention can be implemented even by using aninternal pressure of an exhaust pipe that is located on the upstreamside of the DPF. Besides, an intake air flow amount or an exhaust flowamount may be directly measured for use in the present invention

Further, although the engine acceleration time is used as a transitionaltime for detecting the state of the DPF 102 in the above-describedembodiments, the present invention can be implemented even if adeceleration time is used alternatively.

Besides, although the diesel engine has been described in theabove-described embodiments, a gasoline engine can be applicableequivalently to the present invention.

1. An apparatus for detecting a state of a particulate filter providedin an exhaust system of an internal-combustion engine, the apparatuscomprising: means for determining values of a parameter correlated withflow amount of exhaust gas of the engine; means for detecting adifferential pressure at upstream and downstream sides of theparticulate filter; and means for comparing the ratio of a variationrate of the parameter correlated with the exhaust flow amount to avariation rate of the differential pressure with a predeterminedthreshold value during a transitional operation of the engine todetermine the state of the particulate filter.
 2. The apparatus asclaimed in claim 1, wherein the parameter correlated with the exhaustflow amount is an internal pressure of an air intake pipe of the engine.3. The apparatus as claimed in claim 1, wherein said means for comparingcompares said ratio with a first threshold value to determine a failureof the particulate filter.
 4. The apparatus as claimed in claim 1,wherein said means for comparing compares said ratio with a secondthreshold value to determine that particulates have over-accumulated inthe particulate filter.
 5. The apparatus as claimed in claim 3, whereinthe first threshold value is established in accordance with anaccumulation amount of the particulates in the particulate filter. andthe second threshold value is established in accordance with an amountof residue ashes in the particulate filter.
 6. The apparatus as claimedin claim 4, wherein the second threshold value is established inaccordance with an amount of residue ashes in the particulate filter. 7.The apparatus as claimed in claim 1, wherein said means for comparingincludes means for integrating differences between a current value and aprevious value of said ratio, and wherein the particulate filter isdetermined to be in failure when the integrated value of the differencesis equal to or larger than a predetermined threshold value.
 8. A methodfor detecting a state of a particulate filter provided in an exhaustsystem of an internal-combustion engine, comprising: determining valuesof a parameter correlated with flow amount of exhaust gas of the engine;detecting a differential pressure at upstream and downstream sides ofthe particulate filter; and comparing the ratio of variation rate of theparameter correlated with the exhaust flow amount to a variation rate ofthe differential pressure with a predetermined threshold value during atransitional operation of the engine to determine the state of theparticulate filter.
 9. The method as claimed in claim 8, wherein theparameter correlated with the exhaust flow amount is an internalpressure of an air intake pipe of the engine.
 10. The method as claimedin claim 8, wherein the step of comparing compares said ratio with afirst threshold value to determine a failure of the particulate filter.11. The method as claimed in claim 8, wherein the step of comparingcompares said ratio with a second threshold value to determine thatparticulates have over-accumulated in the particulate filter.
 12. Themethod as claimed in claim 10, wherein the first threshold value isestablished in accordance with an accumulation amount of theparticulates in the particulate filter.
 13. The method as claimed inclaim 11, wherein the second threshold value is established inaccordance with an amount of residue ashes in the particulate filter.14. The method as claimed in claim 8, wherein the step of comparingcomprises: integrating differences between a current value and aprevious value of said ratio; and determining the particulate filter tobe in failure when the integrated value of the differences is equal toor larger than a predetermined threshold value.
 15. A computer readablerecording media storing a computer program for determining a state of aparticulate filter provided in an exhaust system of aninternal-combustion engine, said computer program, when executed,performing: determining values of a parameter correlated with flowamount of exhaust gas of the engine; detecting a differential pressureat upstream and downstream sides of the particulate filter; andcomparing the ratio of variation rate of the parameter correlated withthe exhaust flow amount to a variation rate of the differential pressurewith a predetermined threshold value during a transitional operation ofthe engine to determine the state of the particulate filter.
 16. Themedia as claimed in claim 15, wherein the parameter correlated with theexhaust flow amount is an internal pressure of an air intake pipe of theengine.
 17. The media as claimed in claim 15, wherein said comparingcompares said ratio with a first threshold value to determine a failureof the particulate filter.
 18. The media as claimed in claim 15, whereinsaid comparing compares said ratio with a second threshold value todetermine that particulates have over-accumulated in the particulatefilter.
 19. The media as claimed in claim 17, wherein the firstthreshold value is established in accordance with an accumulation amountof the particulates in the particulate filter.
 20. The media as claimedin claim 18, wherein the second threshold value is established inaccordance with an amount of residue ashes in the particulate filter.21. The media as claimed in claim 15, wherein said comparing comprises:integrating differences between a current value and a previous value ofsaid ratio; and determining the particulate filter to be in failure whenthe integrated value of the differences is equal to or larger than apredetermined threshold value.